Power supplies are well characterized and understood, as all electronic systems require some kind of power. Still, power supply design and selection continues to challenge engineers because of the trend toward smaller size, higher efficiency, higher reliability, and greater power integrity for applications from mobile devices to line powered hardware. With high-speed data communications systems like 5G emerging, timing and noise margin requirements are becoming extreme. To address the problem of efficient and reliable power delivery in smaller form factors, power supply designers may use switch mode power supplies (SMPS) with flyback topology. Such flyback topologies (i.e., circuit designs) can be useful to power levels of up to 150 Watts, and can offer designs with a lower component count at a smaller size and at lower cost. Flyback topologies in circuit designs can also offer input/output isolation, and generally good efficiency.
An SMPS, or “switcher”, is a power source that uses a switching regulator to maintain stable output voltage(s) from an AC or DC source. The switching regulator typically uses one or more semiconductor devices, such as a bipolar junction transistor, metal-oxide-semiconductor field-effect transistor (MOSFET), or insulated-gate bipolar transistor (IGBT), switching between ‘On’ and ‘Off’ states to maintain output voltage regulation. Semiconductor devices can operate with fixed ‘On’ time and variable frequency, or, more commonly, at a fixed frequency and variable duty cycle. High efficiency results from the low power dissipation of the switching device when it is either ‘On’ or ‘Off’. Semiconductor devices generally dissipate power during the transitions between states. Also, because the switching frequency is generally in the tens of kilohertz, transformers, inductors, and capacitors can be much smaller, giving high volumetric efficiency.
Active clamp circuits are being used in switched mode power supplies to recycle leakage inductance losses. For off-line flyback switched mode power supplies, active clamp circuits are typically made with N-type MOSFET switch devices and include a high side driver. High side drivers can add significant cost to active clamp SMPS topologies because high side drivers generally require a level shifting circuit. High side drivers also typically call for an expensive high voltage IC technology. Off-line active clamp flyback (ACF) technology is costly because it typically uses N-type MOSFET switch devices, which are more expensive when compared to bipolar devices.